In general, an image forming apparatus employing an electrophotographic method carries out image forming using a binary developer mixed nonmagnetic toner with a magnetic carrier. It is important to maintain a proper mixing ratio (toner density) between the toner and the carrier for the image forming apparatus to carry out image forming using the binary developer. This is because that if the toner density of the developer in a developing container becomes under a proper value, darkness of image decreases, and that if the toner density exceeds a proper value, such a trouble as so-called fogging happens, which is a phenomenon of tonner's sticking to the part of a recording paper to which toner is not supposed to stick.
For the above reason, it is necessary to accurately detect a toner density and control the toner density to a proper value in order to acquire a fine printed image.
Two toner replenishment controlling methods are available, an inductance detecting method (which is also referred to as a magnetic permeability sensor method) and a video count method (which is also referred to as a dot count method).
The inductance detecting method is the method to control toner replenishment based on the magnetic permeability of a developer that is detected by a popularly used magnetic permeability sensor (toner density sensor).
The video count method is the method to calculate the print ratio of a printed paper (i.e., the ratio of a printed area to the total area of a printed paper), estimate a toner consumption amount based on the calculated print ratio, and control toner replenishment based on the estimated toner consumption amount.
Japanese Laid-Open Patent Publication No. 11-202605 discloses an image forming apparatus that carries out toner density control by the video count method from the start of operation to a point that the number of printed sheets reaches the given number of sheets, and that carries out toner density control by the inductance detecting method after the end of the video counter method.
Japanese Laid-Open Patent Publication No. 04-204468 discloses a toner replenishing device that changes a toner density level for starting toner replenishment by the inductance detecting method, in accordance with a print ratio of a printed paper.
According to the video count method, an operation of a toner replenishing device is calculated based on an estimated toner consumption amount. Since an actual toner consumption amount and an actual toner supply amount per unit operation time of the toner replenishing device change depending on an environmental condition of the device, it sometimes occurres a decrease of darkness of printing that is caused by a decrease in the toner density of a developer resulting from an insufficient toner supply or an excessive contrast of printed image (quality) and toner fogging on a background that are caused by an increase in the toner density of the developer resulting from an excessively ample toner supply.
As described above, proper toner density control is difficult when toner replenishment is controlled only by the video count method.
According to the inductance detecting method, a toner density is measured with a toner density sensor, so that an actual toner density can be kept constant.
An image forming apparatus usually carries out an image quality adjusting processing (so-called process control) to securely keep an image quality in a fine state. This image quality adjusting processing is the processing for forming a reference toner patch on a photosensitive material, detecting the optical reflectance (reflection light density), etc., of the reference toner patch (or the reference toner patch temporarily transferred to an intermediate transferrer), determining a toner sticking amount, etc., on the reference toner patch based on the detected optical reflectance, and adjusting a developing bias condition, etc., based on the determined toner sticking amount.
A conventional image forming apparatus carries out this image quality adjusting processing every time the accumulated number of printed sheets exceeds the given number of sheets. This approach, however, may lead to such a case, depending on the condition of the apparatus, where the image quality adjusting processing is carried out when the processing is unnecessary or where the image quality adjusting processing is not carried out when the processing is necessary. In particular, because the image quality adjusting processing takes time, excessively executing the processing invites a decline in print job efficiency. It is, therefore, preferable that timing to execute the image quality adjusting processing be variable depending on the condition of the apparatus. Japanese Laid-Open Patent Publication No. 11-202605 and Japanese Laid-Open Patent Publication No. 04-204468, however, present no disclosure or suggestion on this respect.